UT Austin team demonstrates new approach to lithium sulfide cathodes for Li-S batteries

2 December 2013

Schematic showing the chemical reduction reaction of one Li2S6 molecule by lithium to form six Li2S molecules, involving the diffusion/driving of lithium out of the graphene layers in the graphite. Credit: ACS, Fu et al. Click to enlarge.

Researchers at the University of Texas at Austin, led by Professor Arumugam Manthiram, have demonstrated that lithiated graphite can serve as a lithium donor in lithium-deficient cathodes used, for example, in high energy density lithium-sulfur chemistry batteries. The lithium in the lithiated graphite chemically reduces in situ the polysulfide Li2S6 in liquid electrolyte to insoluble Li2S as a cathode material for rechargeable Li−S batteries.

The approach offers a new way to introduce lithium into the cathode in Li−S chemistry batteries and potentially could become applicable in lithium metal-free Li−air, or Li−organic batteries as well. A paper on their study is published in the Journal of the American Chemical Society.

Lithium-sulfur (Li−S) batteries—which conventionally use elemental sulfur (with conductive additives) as the cathode, an aprotic liquid electrolyte, and lithium metal as the anode—are attractive because of their high theoretical energy density of 2,567 Wh kg−1, calculated on the basis of the Li anode (∼3,860 mAh g-1) and the S cathode (∼1,675 mAh g-1).

Sulfur has been studied as a cathode material for Li-S batteries for more than 50 years; persistent challenges include low electrical conductivity; dissolution of lithium polysulfides into the liquid electrolyte; and the shuttle effect of these soluble species.

Li2S has a theoretical capacity of 1,166 mAh g-1—nearly 1 order of magnitude higher than traditional metal oxides/phosphates cathodes. If paired with Si anodes with 2,000 mAh g-1 capacity, the specific energy of a Li2S-based lithium-ion battery could be 60% higher than the theoretical limit of metal oxide/phosphate counterparts. (Earlier post.)

Recent efforts, e.g., sulfur−carbon composites, core−shell structures, and trapping polysulfides within the cathode region, have shown significant performance improvements that are close to practical applications. However, the lack of lithium in the sulfur cathode requires the use of a lithium metal anode, which poses serious safety hazards and may not be practical.

Some efforts have recently been focused on the development of lithium sulfide (Li2S) cathodes, which could lead to lithium metal-free Li−S batteries. The intrinsic properties of Li2S, e.g., high resistivity and high reactivity in air, make it highly challenging in fabricating practical Li2S cathodes.

… Lithium polysulfides (Li2Sn, n > 2) are intermediate compounds between elemental sulfur and Li2S, which can be generated in liquid electrolytes by the reaction (n − 1)S + Li2S → Li2Sn and used as cathode materials. For example, the polysulfide Li2S6 can be electrochemically converted to sulfur/high-order polysulfides during charge (Li2S6 → 6S + 2Li+ + 2e−) or insoluble Li2S during discharge (Li2S6 + 10Li+ + 10e− → 6Li2S) in a Li/polysulfide cell. On the other hand, lithium can be electrochemically intercalated into graphite to form LiC6 and reversibly deintercalated.

The lithium stored in the lithiated graphite with a chemical potential close to that of metallic lithium would be a strong reducing agent for chemical reactions, but it has not yet been explored. Herein, we demonstrate that lithiated graphite can donate lithium to chemically reduce Li2S6 in situ in liquid electrolytes to insoluble Li2S, serving as a starting cathode material in Li/polysulfide cells.

—Fu et al.

The team used graphite-containing carbon paper (CP) to fabricate a Li/polysulfide cell with the dissolved polysulfide Li2S6 electrolyte added into the cathode side. The large voids in the carbon paper accommodate the polysulfide electrolyte introduced.

The huge potential difference between the lithiated carbon paper and the L2S6 electrolyte results in a spontaneous chemical reduction of Li2S6 when they are in contact with each other, involving the diffusion/consumption of lithium out of lithiated graphite.

Applied in a half cell, the Li2S-CP electrode showed a uniform distribution of formed sulfur compounds. In the cycled electrode, the reduced sulfur compounds are deposited on both the carbon fibers and spaces between them. The half cells exhibited a stable discharge capacity of ∼800 mAh g−1 over 50 cycles. The Coulombic efficiency starts at ∼80% in the first cycle, increases to over 90% in the second cycle, and then stabilizes. Capacity obtained is largely determined by the structure of the electrode.

… this study, for the first time, shows the
utilization of lithium in lithiated graphite to chemically reduce
in situ the polysulfide Li2S6 in liquid electrolyte to insoluble Li2S as a cathode material for rechargeable Li−S batteries. This chemical reduction reaction process is evident from the distinct interplanar spacing changes in graphite, evolution of the electrochemical behavior of the electrode, and compositional analysis of the reduced sulfur compounds in the electrode. The formed Li2S shows low overpotential in the first charge and good cyclability. This approach offers a new way to introduce lithium into the cathode in Li−S batteries and potentially become applicable for other lithium-free or lithium-deficient electrode materials, e.g., oxygen and organic cathode materials. Moreover, the lithiated graphite could be a lithium source for organic synthesis reactions.